HAP1 (huntingtin associated protein 1) is a neuronal protein that functions primarily in intracellular trafficking and vesicular transport. HAP1 specifically associates with huntingtin (HTT), with this binding enhanced by expanded polyglutamine repeats in mutant HTT 1. The protein plays crucial roles in vesicular transport within neurons and axons, linking HTT to motor proteins and transport cargos 1. HAP1 is involved in multiple trafficking processes including endocytosis of membrane receptors, axonal transport of BDNF, delivery of GABA receptors to synapses, and regulation of autophagosome motility 1. The protein shows differential expression patterns, being predominantly found in the brain but also present in the digestive system, where it exhibits regional variation with highest expression in the stomach and lowest in the intestine 2. Interestingly, HAP1 expression is downregulated in pancreatic cancer, suggesting potential diagnostic value 2. HAP1 has been utilized as a model system in functional genomics research, particularly the near-haploid HAP1 cell line which has become valuable for CRISPR-Cas9 gene editing studies due to its single genome copy allowing unmasked phenotype expression 3. The protein's dysfunction is implicated in Huntington's disease pathogenesis through altered vesicular transport mechanisms.